1. Field of the Invention
The invention relates to a process for synthesis of esters of acrylic and methacrylic acid, especially esters of higher alcohols and substituted alcohols, by transesterifying esters of acrylic and methacrylic acid with alcohols that are readily available on the large industrial scale, in the presence of mixed catalysts containing alkali metal cyanates.
2. Discussion of the Background
Methacrylic acid esters or acrylic acid esters are usually obtained by the reaction of alcohols with simple methacrylic acid esters such as methyl methacrylate or ethyl acrylate. Alkaline catalysts such as lithium and calcium hydroxides are employed for this purpose, while metal catalysts such as titanium alcoholates or even organotin compounds (German Patents 3423441, 3423443, Rxc3x6hm GmbH) may also be used. Alkaline catalysts have the advantage of being easily separated from the product, since they can be removed without previous separating processes such as filtration. Their price largely depends upon their lithium content, and so inexpensive catalyst components would be desirable.
Alkali metal cyanides are also known as selective transesterification catalysts (HoubenWeyl, Methods of Organic Chemistry, Volume 5, pages 702 to 703, Thieme Verlag, 1985). Their toxicity, however, limits their possible industrial uses. The less toxic alkali metal cyanates are suitable for transesterification catalysis only in special cases, namely when reactive substrates such as glycidol or carbonate esters are used. For example, German Patent 2525026 (Degussa AG) describes the synthesis of glycidyl methacrylate by transesterification of methyl methacrylate with glycidol in the presence of a transesterification catalyst and a polymerization inhibitor. Potassium cyanide, potassium cyanate and potassium thiocyanate are used as the transesterification catalyst.
Japanese Patent 55094380 also describes a process for synthesis of glycidyl methacrylate by transesterification in the presence of potassium cyanate. The yield of glycidyl methacrylate is 83.5%.
European Patent 683163 describes the synthesis of glycidyl (meth)acrylate by transesterification of methyl (meth)acrylates in the presence of a polymerization inhibitor and a catalyst. The catalyst is synthesized in situ from an ammonium salt or phosphonium halide and potassium cyanide, cyanate or thiocyanate. At the end of the reaction, the catalyst must be deactivated by the addition of alkali metal or alkaline earth metal salts of a sulfonic acid or of a heteropolyacid to the reaction mixture.
Accordingly, it is an object of the present invention to provide a process for preparing esters of acrylic acid or methacrylic acid, comprising transesterifying an acrylic or methacrylic ester of a C1 to C4 alcohol with a different alcohol, in the presence of a mixed-catalyst system comprising a), an alkali metal cyanate or an alkali metal thiocyanate; and b), an alkaline earth metal oxide, an alkaline earth metal hydroxide, or an alkali metal halide. The process of the present invention avoids the disadvantages of the prior art, in that toxic alkali metal cyanides are not used, and there is no need for additional reaction steps or the addition of materials to deactivate the catalyst. Thus, the process of the present invention is simple, and does not require special deactivation steps which tend to reduce the yield of the special esters formed in the transesterification. In regard to the industrial feasibility of the process of the present invention, the catalyst system of the present invention can be easily separated from the crude ester after the completion of the reaction.
The present inventors have found that, in the presence of a mixed-catalyst system comprising a mixture of a), an alkali metal cyanate or alkali metal thiocyanate, and b), an alkali metal or alkaline earth metal salt, alcohols may be converted to the corresponding acrylic or methacrylic acid esters with high purity and with few byproducts, by transesterification with acrylic or methacrylic acid esters of a C1 to C4 alcohol.
Examples of suitable alkali metal cyanates are sodium cyanate or potassium cyanate, and examples of suitable alkali metal thiocyanates are sodium thiocyanate or potassium thiocyanate. The alkali metal cyanates or the alkali metal thiocyanates can be used individually or as mixtures.
The alkali metal salt may be, for example, an alkali metal halide such as sodium chloride, potassium chloride, sodium fluoride, potassium fluoride, sodium bromide, potassium bromide, sodium iodide, lithium chloride, lithium bromide or potassium iodide. The alkaline earth metal salt may be, for example, an alkaline earth metal oxide such as magnesium oxide, calcium oxide or barium oxide. The corresponding hydroxides may also be used.
If the catalyst is a mixture of alkali metal cyanate and alkaline earth metal oxide, the weight ratio of alkali metal cyanate to alkaline earth metal oxide may be 10:1 to 1:2, and may include any range or ratio therebetween, including 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, and 1:1. If the catalyst is a mixture of alkali metal cyanate and alkaline earth metal hydroxide, the weight ratio of alkali metal cyanate to alkaline earth metal hydroxide may be 50:1 to 1:2, and may include any range or ratio therebetween, including 45:1, 40:1, 35:1, 30:1, 25:1, 20:1, 15:1, 10:1, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, and 1:1. If the catalyst is a mixture of an alkali metal cyanate and an alkali metal halide, the weight ratio of the alkali metal cyanate to alkali metal halide may be 5:1 to 1:1, and may include any range or ratio therebetween, including 4:1, 7:2, 3:1, 5:2, 2:1, and 3:2.
The amount of catalyst which may be added to the feed mixture of the C1 to C4 alcohol ester of acrylic or methacrylic acid and the different alcohol to be transesterified, ranges from 0.5% to 5% of the feed mixture, and may include all values and subranges therebetween, such as 1%, 1.5%, 2.5%, 3%, 3.5%, 4%, and 4.5%.
The acrylic or methacrylic acid esters of a C1 to C4 alcohol may include methyl acrylate or methacrylate, ethyl acrylate or methacrylate, propyl acrylate or methacrylate, isopropyl acrylate or methacrylate, butyl acrylate or methacrylate, isobutyl acrylate or methacrylate, sec-butyl acrylate or methacrylate, and tert-butyl acrylate or methacrylate.
In order to provide an industrially feasible process, it is advantageous if the catalyst can be easily separated from the crude ester product without additional process steps after completion of the reaction. A particularly desirable separation process is filtration. A further advantage of the catalyst system of the present invention, compared with the known titanium catalysts, is the water tolerance of the catalyst system of the present invention. The titanium catalysts are deactivated immediately by water in the reaction system, whereas the catalyst system of the present invention is not.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.
Apparatus
The feed materials, comprising the alcohol to be transesterified and methyl methacrylate (MMA), was introduced into an apparatus comprising a 0.5-liter three-necked flask with mechanical stirrer, air inlet and top-mounted packed column (30 cm long, filled with 6 mm Raschig rings) as well as column head with reflux splitter. Hydroquinone monomethyl ether was used for stabilization against premature polymerization, and further inhibitors were added as necessary.
Transesterification Conditions
The feed materials were first heated to boiling with the introduction of air, and the azeotrope of MMA and water formed thereby from the wet feed materials was distilled off above the column until the head temperature was constant at 100xc2x0 C., and clear MMA was distilled over. The bottoms (i.e., the components remaining in the flask that were not distilled off) were then allowed to cool to a temperature of about 90xc2x0 C., and the quantity of MMA removed by distillation was replenished with pure MMA. The catalyst was added, and the contents of the flask were then reheated to boiling. The resulting azeotrope of MMA and methanol formed, distilled over (head temperature: 65 to 99xc2x0 C.). Toward the end of the reaction, the head temperature approached the boiling point of pure MMA (100xc2x0 C. at normal pressure). When only pure MMA was still distilling over, the transesterification was complete. The contents of the flask were then allowed to cool, and the residual MMA was removed under vacuum (about 10 mbar). The catalyst precipitate thus formed was separated by filtration. The filtrate was analyzed by gas chromatography.